The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Aspects of the present invention pertain to a process for the continuous production of polyesters, in particular aliphatic polyesters, from cyclic ester monomers.
Aliphatic polyesters based on cyclic ester monomers such as lactide (L-lactide, D-lactide, rac-lactide, also referred to as DL-lactide, meso-lactide), glycolide, trimethylene carbonate (TMC), epsilon-caprolactone, and p-dioxanone, and combinations thereof have many attractive properties. They often have high biocompatibility and attractive resorbability properties, which makes them suitable for the preparation of scaffolds and implants for use in human or animal bodies, such as for example for fixation elements, films, membranes, suture thread or also for pharmaceutical drug delivery systems. Further, in particular, polylactide, also referred to as polylactic acid, is a promising material in the field of biobased polymers for, e.g., packaging material. The fact that it can be derived from renewable resources makes it particularly attractive as a sustainable alternative for polymers derived from oil.
Polymerisation processes for preparing aliphatic polyesters such as polylactide are known in the art. They include ring-opening polymerisation processes and polycondensation processes. It has been found that the polylactic acid obtained through polycondensation processes is of limited value because polycondensation does not yield the required high molecular weight polyesters.
Accordingly, (co)polyesters are preferably prepared by ring-opening polymerisation of the corresponding cyclic monomers, such as L-lactide, D-lactide, DL-lactide or rac-lactide, meso-lactide, glycolide, trimethylene carbonate, epsilon-caprolactone, and p-dioxanone, or mixtures thereof.
Therefore, most publications on processes for the production of polylactide disclose a first step wherein lactic acid is polymerised to form a pre-polymer through condensation, which pre-polymer is subsequently depolymerised by means of a catalyst to form crude lactide (i.e. the ring-closure reaction). The crude lactide is purified, and the purified lactide is used as monomer in the preparation of polylactide by ring-opening polymerisation. For the purpose of this description the terms polylactide and polylactic acid are used interchangeably.
Although the literature on the production of polyesters such as polylactide is abundant, most publications are silent on the specific equipment to be used on industrial scale. They mainly focus on laboratory scale. In most publications, the preparation of lactide from lactic acid and the subsequent purification of the lactide are described in detail, while for the ring-opening polymerisation of cyclic ester monomer (e.g. lactide) to form the corresponding polyester, e.g., polylactide, only temperature and catalyst are described.
Polymerisation processes for manufacturing polyesters can be divided into two groups, viz. polymerisation in the presence of a water-free solvent, e.g., suspension or emulsion polymerisation, and polymerisation in the substantial absence of solvent, e.g., melt polymerisation, carried out at a temperature above the melting temperature of the monomer and polymer, or mass polymerisation, carried out—in batch—at a temperature below the melting temperature of the polymer.
In general, polyesters are all made in the absence of solvent—bulk—by polycondensation and in some special cases, when the cyclic ester monomer is already dehydrated, by ring-opening polymerisation. Commonly used processes are performed batch-wise and the conversion is followed by monitoring increase of melt viscosity and reduction of carboxylic acid end group concentration. This classic approach is used for many polyesters, from rosin-based printing ink resins, via powder coating polyester resins to prepolymers for PET yarn. The latter are subjected after melt-polymerisation to solid-state post-polymerisation (SSP) in order to increase the average molecular weight to values that are not achievable in the molten state. Although SSP is a time-consuming process, it is inevitably applied broadly on industrial-scale.
A major problem which is often encountered in the polymerisation in the absence of solvent of cyclic ester monomers to polyesters, e.g., the polymerisation of lactide to polylactide, is the removal of the heat generated during the exothermic polymerisation reaction. Polyesters such as polylactide have a relatively low thermal conductivity. For example, the thermal conductivity of polylactic acid is 0.13 W/(m·K). For other polymers and rubbers, values of the same order have been reported. This means that the heat generated by the reaction cannot always effectively be removed, especially in large vessels, stirred tanks, and the like. This can lead to local overheating of the resulting polymer, causing chain degradation and discolouration of the polymer. In conventional processes the process settings are chosen such that reaction rates are low and residence times are long. This leads to voluminous and expensive equipment. Other conventional processes do not provide high quality, i.e. high molecular weight, polyester, e.g., polylactide, with a low yellowness index with low residual lactide content in high yields.
A further problem which is encountered in the solvent-free ring-opening polymerisation of cyclic ester monomers is the difference in melt viscosity between the monomer melt and the polymer melt. In WO 99/50345 it is suggested to use a plug flow reactor or a series of plug flow reactors for the polymerisation process. We have found that when using a plug flow reactor, for example for lactide polymerisation, the difference in melt viscosity between the molten cyclic ester monomer (in this example the lactide), and the resulting polyester (in this example the polylactide) is so high that plug flow conditions cannot be maintained and channelling occurs.
Since no appropriate solutions to the problems mentioned above have been disclosed, there remains a need for a continuous process for manufacturing high molecular weight polymers in high quality from cyclic ester monomers in an economically attractive manner on industrial scale.